1. Field of the Invention
This invention relates to a catadioptric reduction projection optical system and method for use, for example, in an exposure apparatus for the manufacture of semiconductive elements, and particularly suitable for application to an optical system for reduction-projecting a pattern more enlarged than the pattern of a real element.
2. Related Background Art
Semiconductive integrated circuits have become more and more minute and exposure apparatuses for printing the patterns thereof are required to have higher resolving power. To satisfy this requirement, the wavelength of a light source must be made short and the numerical aperture (N.A.) of an optical system must be made great. However, if the wavelength becomes short, glass materials standing practical use will become limited because of the absorption of light. If the wavelength becomes 300 mm or shorter, what can be practically used will be only synthetic quartz and fluorite (calcium fluoride). Also, fluorite is bad in temperature characteristic and cannot be used in a great quantity. Therefore, it is very difficult to make a projection lens of a refracting system alone. Further, because of the difficulty of aberration correction, it is also difficult to make a projection optical system having a great numerical aperture of a reflecting system alone.
So, there have been proposed various techniques of constructing a projection optical system by combining a reflecting system and a refracting system. An example of such techniques is a ring field optical system as disclosed in U.S. Pat. No. 4,747,678. In this optical system, an off-axis light beam is used so that incident light and reflected light may not interfere with each other, and the design is made such that only an off-axis zonal portion is exposed to light.
As another example, a projection exposure apparatus comprising a projection optical system having a beam splitter disposed therein, and a catadioptric system for collectively projecting the image of a reticle (mask) by an on-axis light beam is disclosed, for example, in U.S. Pat. Nos. 3,698,808 and 4,953,960.
FIG. 5 of the accompanying drawings schematically shows the optical system disclosed in U.S. Pat. No. 4,953,960. In FIG. 5, a light beam from a reticle 21 on which a pattern to be reduction-transferred is depicted is converted into a substantially parallel light beam by a lens unit 22 having positive refractive power and is applied to a prism type beam splitter (beam splitter cube) 23. The light beam transmitted through the joint surface 23a of this beam splitter 23 is diffused by a correction lens unit 24 having negative refractive power and is reflected by a concave reflecting mirror 25. The light beam reflected by the concave reflecting mirror 25 passes through the correction lens unit 24 again and is reflected by the joint surface 23a of the beam splitter 23, whereafter it is converged on a wafer 27 by a lens unit 26 having positive refractive power, and the reduced image of the reticle pattern is formed on the wafer 27. An example in which a half mirror comprising a plane parallel plate is used instead of the prism type beam splitter 23 is also disclosed.
In the example of the prior art shown in FIG. 5, the reduction magnification of the entire system is 1/4 and the magnification in the concave reflecting mirror 25 is 0.287. Also, the magnification of the concave reflecting mirror in the example of the construction using the obliquely disposed plane parallel plate having a half-transmitting surface is 0.136. That is, in the example of the prior art, the design is made such that with the burden of reduction magnification cast on the concave reflecting mirror, aberrations attributable to a concave reflecting mirror of small reduction magnification are corrected by the correction lens unit 24 and the lens unit 26.
In the ring field optical system according to the prior art, however, it is difficult to make the numerical aperture great. Moreover, it is also impossible to expose collectively and therefore, it is necessary to effect exposure while moving the reticle and the wafer at different speeds correspondingly to the reduction ratio of the optical system, and this has led to the inconvenience that the construction of the mechanical system becomes complex.
Also, in the construction disclosed in U.S. Pat. No. 3,698,808, there is the inconvenience that the flare by the reflection on the refracting surface of the optical system subsequent to the beam splitter is great. Further, characteristics such as the reflectance irregularity, absorption and phase change of the beam splitter are not at all taken into account and therefore, the resolving power is low and the magnification of the entire system is one-to-one magnification, and the apparatus of the prior art cannot possibly stand the use as a semiconductor manufacturing exposure apparatus of the coming generation of which higher resolving power is required.
Furthermore, in the projection optical system disclosed in U.S. Pat. No. 4,953,960, almost all of the reduction magnification of the entire system is borne by the concave reflecting mirror, and this leads to the inconvenience that spherical aberration created by the concave reflecting mirror is great. Accordingly, an optical system for correcting that spherical aberration becomes complicated. Also, since design is made such that the light beam from the reticle 21 is converted into a substantially parallel light beam by the lens unit 22 of positive refractive power, the spacing between the reticle 21 and the beam splitter 23 becomes long, and this leads to the bulkiness of the optical system.